1. Field of the Invention
The present invention relates to a photographic film cassette, more particularly to an improvement of a photographic film cassette in which rotation of a spool core causes a film leader to advance to the outside of a cassette shell.
2. Description of the Related Art
A conventional film cassette includes 35 mm photographic film positioned therein so that the film leader does not protrude from the cassette shell prior to loading the film cassette in a camera. Such a film cassette is easily loaded onto a camera. Simple feeding mechanisms are used with this type of film cassette and include a construction which rotates the spool core to unwind the photographic film, thereby causing the film leader to move through a film passage slit and exit from the film cassette Examples of such a conventional film cassette are disclosed in U.S. Pat. Nos. 4,832,275, 4,834,306, 4,846,418 and 4,883,236 and Japanese Patent Laid-open Publication No. 3-37645.
In devices disclosed by the above-mentioned references, the roll of photographic film wound about the spool core is prevented from loosening in order to transmit rotation of the spool core to the film leader. In U.S. Pat. No. 4,832,275 for example, a pair of rotatable rings are provided on the cylindrical surface of the inside of the cassette shell so as to abut the outermost turn of the roll of film. In Japanese Patent Laid-open Publication No. 3-37645, which has a publication date after the priority date of the present application, a pair of annular ridges are formed on lateral surfaces of the inside of the cassette shell in order to press the spool flanges inwardly so that the roll of film is clamped between the spool flanges In U.S. Pat. No. 4,834,306, circumferential lips are formed around the spool flanges so as to abut the outermost turn of the roll. To direct the film leader toward the outside of the cassette shell, the film cassette of U.S. Pat. No. 4,834,306 has a pair of spreading projections formed inwardly from the film passage slit for deforming and spreading the spool flanges so as to release the outermost turn from circumferential lips.
To produce the cassette shell of such conventional designs, molding resin, e g , thermoplastic resin, is very advantageous as it is convenient and inexpensive. Japanese Patent Laid-open Publication Nos. 50-33831 and 57-190948 disclose that it is desirable to construct the cassette shell from a plurality of, e.g., two, molded shell components. Junction of these shell components is performed by the use of ultrasonic welding or application of an adhesive agent. The ultrasonic welding is particularly advantageous because it is less laborious than applying the adhesive agent, effective for a large area to be joined, and suitable for mass production.
However, unnecessary welding might take place on surfaces of the spool core or the spool flanges in contact with the cassette shell while the shell components are joined by applying ultrasonic waves. This is because the spool flanges or the circumferential lips are in constant contact with the annular ridges or the spreading projections with mutual pressure for the purpose of propelling a film leader. In the case of such undesired welding, the film leader can not be properly propelled even when the spool core is rotated in the film unwinding direction.
Moreover, contact between the above-mentioned members involves considerable friction which enlarges the necessary torque to be applied to the spool core for rotating it. Friction of lateral sides of the photographic film against the spool flanges or the circumferential lips also increases the torque necessary for rotating the spool core.
There is a proposal, as disclosed in Japanese Patent Laid-open Publication Nos. 62-286043 and 2-264941, to reduce the influence of friction in a film cassette of the conventional type between the spool core and its bearing holes or between the spool flanges and the end faces of the roll of film. This proposal is based upon an improvement of the resin for molding the spool. It is, however, difficult in these film cassettes of leader-propelled type, to effectively reduce the friction between the relevant members by the sole use of the improved material as disclosed in these references.